The incident occurred as the Portuguese star prepared for the match against Young Boys at their Wankdorf stadium on Tuesday night.
Practicing shots from range, the all-time Champions League top scorer misfired and hit the poor steward by accident, according to pitchside reports.
The steward immediately fell to the ground, and joining others to check all was OK, Ronaldo immediately went over to the injured party and spent some time explaining himself as fans recorded him.
Then, once he knew the steward seemed to be alright, he ran back onto the turf to continue warming up.
Following on from a dream return to Old Trafford where he bagged two goals in a 4-1 win over Newcastle United at the weekend, Ronaldo was clearly unshaken by the incident.
Needing just 13 minutes to make his mark, he met a superb cross from compatriot Bruno Fernandes that was buried past a helpless David Von Ballmoos.